The present invention relates to working by forging and pressing, and, more particularly, it relates to a method of controlling hydraulic screw presses and apparatus capable of performing such a method.
Over the recent years, presses of the hydraulic-screw type have been finding an ever wider field of applications, owing to their offering an advantage over other previously known pressing machinery, residing in their inherent capacity of having the impact energy regulated within a relatively wide range, i.e. of metering-out the impact energy.
This advantage provides for more accurate dimensions of the shaped work, and also reduces the wear of the tooling and prolongs the life of the pressing machines.
While effecting the operation of a hydraulic screw press, in most cases the ram is driven, first, through the approach path or portion of its stroke, whereafter the ram is accelerated to a required speed, with the acceleration terminated upon the ram having attained the required speed corresponding to the required impact energy.
One of the disadvantages of this known control method is inadequate accuracy of metering-out the impact energy, which is explained by the fact that upon the working fluid distributing device having been turned off following the acceleration, the speed of the ram decreases. This takes place on account of the cutting-off of the supply of the working fluid being effected upon the ram attaining the required speed at a certain distance from the point of the engagement between the tool and the work.
The ram has to pass this distance by the momentum gained (i.e. by the kinetic energy stored by the movable portions of the press), while the working fluid is not supplied into the hydraulic drive cylinder of which the movable element follows the motion of the ram. This results in the suction being created, and the liquid stream breaking somewhere in the line communicating the distributing device with the cylinder, whereby the ram is noticeably braked over this portion of its path.
Consequently, the speed or velocity of the ram at the moment of delivering an impact upon the work would differ from the preset one, this actual final speed of the ram being dependent predominantly on the length of its path after the termination of the supply of the working fluid.
However, as experiments have proved, the point in the path whereat the ram attains a required speed is not a permanent one, but is prone to shift in accordance with various factors such as variation of the viscosity of the working fluid, varying press lubrication conditions, pressure fluctuations in the pressure accumulator, and so on. This results in a varying distance between the point of deenergization of the drive and the point of impact, whereby the final speed of the ram varies.
Hence, the attained impact energy differs from the preset one, and the accuracy of metering-out the energy is inadequate.
The above described method is performed by apparatus including a speed sensor or transmitter connected via mechanical gearing to the ram of the press, and a speed comparison unit having its inputs connected to the master speed control and the speed sensor. The output of the speed comparison unit is connected to the solenoid controlling the distributing device in the hydraulic line connecting the accumulator with the hydraulic drive cylinder of the press.
However, this apparatus is not proof against the shortcomings of the abovedescribed method.